The present invention relates to mobile telecommunications. More particularly, and not by way of limitation, the present invention is directed to a system and method for selecting power-efficient Enhanced Uplink Transport Format Combinations (E-TFCs) for power control of uplink transmissions.
Conventional mobile communication systems set up communication among multiple mobile terminals (for example, User Equipment (UE) devices) and base stations on a multitude of channels, where uplink transmissions are transmissions from the UE to the base station and downlink transmissions are transmissions from the base station to the UE. Some signaling protocols specify uplink transmissions on more than one channel, such as a control channel and a data channel. Thus, transmitter circuitry of the UE may transmit on one or more adjacent channels, possibly leading to adjacent channel interference. Accordingly, it is necessary to control the transmitter circuitry to avoid such issues.
In wide-band CDMA (WCDMA), the transmitter circuitry can be controlled to reduce adjacent channel interference problems by performing rate selection, which includes selecting the data rate and coding scheme (also known as Transport Format Combination (TFC) selection) for a signal transmission or burst to control the transmitter's power amplifier. In newer specifications for WCDMA, there are at least five channels that need to be supported for uplink transmission: the Dedicated Physical Control Channel (DPCCH), the Dedicated Physical Data Channel (DPDCH), the High Speed Dedicated Physical Control Channel (HS-DPCCH), the Enhanced Dedicated Physical Control Channel (E-DPCCH), and the Enhanced Dedicated Physical Data Channel (E-DPDCH). The rate selection for the enhanced uplink channels (E-DPCCH and E-DPDCH) is referred to as enhanced transport format combination (E-TFC). Each channel has a corresponding gain factor (βc, βd, βhs, βec, βed) determining the power offsets between the channels, and each channel has a corresponding spreading factor, I or Q branch assignment, and channelization code. In addition, there can be 1, 2, or 4 E-DPDCHs. Many thousands of different transmitter configurations are possible given the range of values possible for the gain parameters, number of codes, and spreading factors.
In the 3GPP release 99, the Radio Network Controller (RNC) controls resources and user mobility. Resource control in this context relates to admission control, congestion control, and channel switching (roughly changing the data rate of a connection). Furthermore, a dedicated connection is carried over a Dedicated Channel (DCH), which is realized as the DPCCH and the DPDCH.
In the evolved 3G standards, the trend is to decentralize decision making, and in particular the control over the short term data rate of the user connection. The uplink data is allocated to an Enhanced Dedicated Channel (E-DCH), which operates similarly to the DCH. The E-DCH is realized as the DPCCH, which is continuous; the E-DPCCH for data control; and the E-DPDCH for data. The two latter channels are only transmitted when there is uplink data in the send buffer to send. Hence the Node B uplink scheduler determines which transport formats each user may use over the E-DPDCH. The RNC, however, is still responsible for admission control.
FIG. 1 is a simplified functional block diagram of a conventional uplink power control mechanism 10. Uplink power control is divided between an inner loop power control mechanism 11 and an outer loop power control mechanism 12. A receiver 13 receives a baseband signal 14 and generates a DPCCH signal-to-interference ratio (DPCCH_SIR) 15. The DPCCH_SIR is input to the inner loop power control mechanism 11. The inner loop power control mechanism is a conventional DCH power control mechanism. The outer loop power control mechanism 12 adjusts a DPCCH_SIR target 16 to ensure that the E-DPDCH is operating at the correct power level by monitoring the number of retransmissions in a Hybrid Automatic Repeat Request (HARQ) receiver (not shown). The DPCCH_SIR target is input to the inner loop power control mechanism 11, which compares the target with the DPCCH_SIR received from the receiver to produce power control (PC) commands 17. Thus, the E-DPDCH is maintained by outer loop power control, while the DPCCH quality is related to the offset βed/βc between DPCCH and DPDCH.
For the DCH, the outer loop power control mechanism 12 adjusts the DPCCH_SIR target 16 to ensure that the DPDCH operates at the correct power level by monitoring whether or not the transport blocks are correctly received.
The data transmission configuration over E-DPDCH is predefined as a number of E-TFCs. Each E-TFC is associated with a number of data blocks, each with 320 data bits; a spreading factor; a number of scrambling codes; a power offset, which determines the E-DPDCH power relative the DPCCH power; a code rate for each transmission and subsequent retransmissions, if needed; and the like. Many aspects must be considered when determining how these parameters should be configured. The power offsets per E-TFC may be signaled to the UE, or may be computed by the UE. If the UE computes the E-TFC power offsets, the computation is based on the signaled power offsets of a set of reference E-TFCs.
The Node B scheduler allocates resources to UEs by signaling a maximum allowed power offset over the Absolute Grant Channel (AGCH). By comparing this maximum allowed power offset to the power offset per E-TFC, this restricts which E-TFCs the UE is allowed to use. The available power in the UE and the amount of data in the UE send buffer may further restrict which E-TFC that the UE will use when transmitting data.
The required E-DPDCH power to fulfill the outer loop depends primarily on the data rate and the code rate. The power increases with the data rate if the code rate is constant, and the power increases with the code rate. When transport formats are configured in a way that uses high code rates (>0.5), these transport formats provide relatively little data protection in terms of coding, and require relatively high signal quality at the receiver.
If the (signaled or calculated) power offset between the DPCCH and the E-DPDCH for a particular transport format is insufficient, it leads to many retransmissions. In response, the DPCCH_SIR target is increased in the outer loop power control, and therefore it has an impact on the transmitted power of all channels, including both the E-DPDCH and DPCCH, during the time period after the high code rate was used until the outer loop power control has converged to a lower level again. The increased DPCCH_SIR target also causes too many retransmissions of the current transport block since the outer loop is at a level too low for the E-TFC. The block error rate (BLER) also increases, as does retransmission at the RLC layer, which further increases the delay and the used power per transport block. This has a negative impact on the required power per bit for successful communication.
FIG. 2 is a graph of relative energy per kilobit for the E-DPDCH and DPCCH as a function of the EUL bit rate when utilizing a conventional power control mechanism. The energy per kilobit is shown for different transport formats relative to the first transport format. If the code rate is held roughly equal in the different transport formats, the expected behavior would be a monotonically decreasing curve, since the cost of the DPCCH would diminish as the data rate and the power increases. Instead, transport formats with high code rates are identified as relatively power inefficient.
FIG. 3 is a conventional graph of code rate as a function of the EUL bit rate. FIG. 3 illustrates that as the code rate increases, the EUL bit rate increases. At a code rate of approximately 0.75, another code is added, reducing the code rate to approximately 0.4. Thereafter, with increasing code rate, the EUL bit rate increases, but at a lower rate due to the additional code. A disadvantage of adding additional codes is that the complexity and cost of the base station is correspondingly increased.
It would be advantageous to have a system and method that overcomes the disadvantages of the prior art by avoiding the power inefficient transport formats for data transmission. The present invention provides such a system and method.